In the field of product labeling, it has been common practice to apply the appropriate parameters such as content, weight, price and the like to the labels by means of printing apparatus utilizing ink or ink ribbons. It is further common practice to print machine readable indicia such as the bar code (now in use on the vast majority of products) on the product label by means of conventional ink printing apparatus. Meanwhile, the use of thermal printing on product labels has greatly increased in the manner of providing clear and well-defined printed characters and/or images.
The machine readable and human readable printing by use of thermal elements also has been expanded into the area of perishable goods which may be packaged in soft packages and stored in an adverse atmosphere that may affect the printing on the package. The wrapped products may include meat, poultry, fish, produce or the like which are subject to an environment containing water or water vapor (condensation) animal fat, oil, vinegar, blood, and alcohol, and it is commonly known that the printing on the labels for these products must be protected from exposure to such environmental elements to enable fast and correct reading of the printed matter.
Representative documentation in the field of protection for thermosensitive type sheets includes U.S. Pat. No. 4,370,370, issued to S. Iwata et al. on Jan. 25, 1983, and which discloses a thermosensitive recording adhesive label having a support sheet, a coloring layer of leuco dye and acidic material on the front side of the support sheet, a front barrier layer of polymeric material, a layer on the back barrier layer, and a disposable backing sheet peelable from the adhesive layer.
U.S. Pat. No. 4,388,362, issued to S. Iwata et al. on June 14, 1983, discloses a heat sensitive recording paper having a substrate, a color forming layer of leuco dye and acidic material on the substrate, and a protective layer of water soluble resin. A pattern is printed on the protective layer with ultraviolet setting type ink and an adhesive layer is formed on the back of the substrate with a releasable paper on the adhesive layer.
U.S. Pat. No. 4,424,245, issued to K. Maruta et al. on Jan. 3, 1984, discloses a thermosensitive recording type label sheet having a support, a coloring layer of leuco dye and acidic material on the front side, a barrier layer of water soluble polymeric material and water repellent wax material, and an adhesive layer on the barrier layer.
U.S. Pat. No. 4,426,422, issued to G. R. E. Daniels on Jan. 17, 1984, discloses distortion and chemically resistant heat transfer materials formed by a mixture of two interspersed polymers, one being an acid based polyester and the other an ethylene vinyl acetate copolymer. The labels resist alcohols, oils, detergents, inks and adhesives.
U.S. Pat. No. 4,444,819, issued to K. Maruta et al. on Apr. 24, 1984, discloses thermosensitive recording material having support material, a coloring layer of leuco dye and acidic material, and a protective layer of PVA with a saponification ratio of 70 to 85%.